The present invention relates to closure assemblies for electrochemical cells and particularly to a rupturable vent seal and apparatus adapted for use in non-aqueous electrochemical cells.
Batteries used as power sources for electronic equipment need to store large amounts of energy. Such batteries may contain one or more electrochemical cells. Pressure inside each cell may increase due to changes in internal temperature, an increase in internal volume of electrodes during discharge and/or gases generated during cell charging (in the case of secondary batteries) and/or discharge of the cell. Thus, batteries typically include a mechanism for releasing or discharging gas away from the cell to limit the buildup of internal pressure.
Some electrochemical battery cell designs have an open-ended container and a collector assembly disposed at the open end of the container to close the electrochemical battery cell. In such cases, the collector assembly (also sometimes referred to as a header assembly) usually includes a safety pressure release vent mechanism that releases excessive pressure.
Primary and secondary electrochemical cells having nonaqueous electrolytes, such as cells with electrodes containing lithium metal and lithium intercalation materials, typically have collector assemblies with thin-walled plastic sealing members to minimize vapor transmission and pressure relief vents that are able to very quickly reduce internal pressure. While issues associated with the caustic aqueous electrolyte solutions are relatively well documented, the variety, volatility and reactivity of the various non-aqueous electrolytes and salts required by the specific chemistries used in such cells pose a unique set of challenges for cell designers, in terms of materials, design specifications and the like.
Various collector assembly and pressure release vent designs have been used in electrochemical cells. For example, resealable pressure relief vents can be found in rechargeable aqueous electrolyte cells, such as nickel-cadmium and nickel-metal hydride cells. Primary (nonrechargeable) aqueous cells, such as alkaline zinc-manganese dioxide cells, have used collector assemblies with relatively large surface area plastic seals containing a weak section that can rupture when the internal pressure exceeds a predetermined limit.
FIG. 1 shows a cross-sectional view of the top portion of a typical electrochemical battery cell 100 of the prior art. The electrochemical battery cell 100 includes a housing 102 that includes a container 104 having a bead 107 that separates the top and bottom portions of the container 104, and an open end that is closed by collector assembly 106. Collector assembly 106 acts as a closure mechanism for the open end of container 104 and includes a positive contact terminal 116 having one or more vent holes 130, a gasket 124, a positive temperature coefficient (PTC) device 126, a cell cover 144, a bushing 146, a vent ball 148, and a contact spring 122 that is in physical contact with the current collector 136 which extends from the electrode assembly (not shown) in the bottom portion of container 104. The current collector 136 is otherwise physically separated from the container 104 by an insulator 138. The cell cover 144 has a vent well 150 that projects downward away from the positive contact terminal 116 internal to the electrochemical battery cell 100. The vent well 150 has a vent aperture 152 formed therein which is sealed by the vent ball 148 and vent bushing 146 when they are seated in the vent well 150 such that the bushing 146 is compressed between the vent ball 148 and the vertical wall of the vent well 150. When the internal pressure of the electrochemical battery cell 100 exceeds a predetermined level, the vent ball 148, and in some cases both the bushing 146 and the vent ball 148, are forced away from the vent aperture 152 and at least partly out of the vent well 150 to release pressurized gas through the vent aperture 152 and vent holes 130 of electrochemical battery cell 100.
Other examples of conventional collector assembly and pressure release vent designs can be found in: U.S. Pat. No. 4,963,446 (issued to Roels et al. Oct. 16, 1990), U.S. Pat. No. 5,015,542 (issued to Chaney, Jr. et al. May 14, 1991), U.S. Pat. No. 5,156,930 (issued to Daio et al. Oct. 20, 1992), U.S. Pat. No. 5,609,972 (issued to Kaschmitter et al. May 11, 1997), U.S. Pat. No. 5,677,076 (issued to Sato et al. Oct. 14, 1997), U.S. Pat. No. 5,741,606 (issued to Mayer et al. Apr. 21, 1998) and U.S. Pat. No. 5,766,790 (issued to Kameishi et al. Jun. 16, 1998). Each of these examples has a large collector assembly volume or dimensional constraints limiting the internal volume within the cell for active ingredients or a large number of components making the cell more costly and difficult to manufacture. In fact, most collector and closure assemblies in the art require a substantial amount of space, in relation to the height of the cell (and mores specifically, in cylindrical cells, in relation to the axial height), thereby reducing the overall amount of electrochemically reactive materials that may be enclosed within the cell. Additionally, reliance upon multiple and/or relatively large-sized gaskets may further limit the ability of a cell to retain its electrolyte because of the vapor-transmissive properties of the gasket material.
Because the vast majority of commercially available primary cells must have containers sized to standardized dimensions (e.g., a “AA” size or, according to ANSI nomenclature, a R6 size container), the ability to volumetrically maximize electrochemically reactive materials within a cell is of the utmost importance, particularly with respect to many of the smaller standardized sizes (e.g., “AAA” size or, according to ANSI nomenclature, a R3 size container). Moreover, volumetric issues are of particular concern in electrochemical systems incorporating a lithium electrode, because many lithium-based systems (and especially those adapted to a 1.5V platform) tend to expand during discharge.
Notwithstanding the desire to maximize volumetric capacity by reducing the volume of the closure assembly—particularly with respect to its profile (i.e., the amount of axial height it requires in a cylindrical cell), cell closure systems must still provide a long-lasting seal that is impervious to corrosion and that reliably prevents leakage and/or vapor transmission of any volatile electrolytes or other fluids contained within the cell both prior and subsequent to discharge. Weight loss concerns are even more significant in smaller standard cell sizes (e.g., R3 and/or “AAAA” size or, according to the ANSI nomenclature, R61 size cells, in particular) because the potential volume and weight of the overall cell decreases more significantly than the surface area through which vapor may be transmitted (i.e., the header assembly) as the volume capacity decreases in standard cell sizes.
Cell closure systems must also allow for emergency venting of volatile fluids during discharge so as to prevent catastrophic failures of the cell. Absent a vent mechanism, the cell may bulge, leak or disassemble during discharge.